1. Field of Invention
The present invention relates to a projector that projects images to display the images, and to a prism used therein.
2. Description of Related Art
In a projector, modulated light is formed by using an electro-optic device, and this modulated light is projected to display an image. Generally speaking, an xe2x80x9celectro-optic devicexe2x80x9d is a device for converting an electric signal representing image information to modulated light. As this electro-optic device, an optical modulator which modulates light by controlling the emitting direction of illumination light applied to each pixel according to image information or the like is used. An example of this optical modulator is a micro-mirror type optical modulator such as the digital micro-mirror device (a trade mark of TI (Texas Instruments, Co.), hereinafter referred to as xe2x80x9cDMDxe2x80x9d).
The DMD has a plurality of micro-mirrors corresponding to a plurality of pixels forming an image. The inclination of each of the plurality of micro-mirrors varies according to image information. The light is reflected according to the inclination of each micro-mirror. Of the light reflected by each micro-mirror, light reflected in a predetermined direction is utilized as the light for forming an image.
FIGS. 11(A)-(D) are diagrams illustrating the DMD and prism in a conventional projector. FIGS. 11(A), 11(B), 11(C) and 11(D) are a front view, a plan view, a right side view, and a perspective view, respectively. A prism 600 is arranged in the proximity of a light illumination surface 502 of a DMD 500. An axis parallel to a central axis 500ax perpendicular to the light illumination surface 502 of the DMD 500 is the z-axis, and the direction from the DMD 500 to the prism 600 is the normal direction. Further, of the axes which are perpendicular to the z-axis and orthogonal to each other, the horizontal axis is the x-axis and the vertical axis is the y-axis.
In the following description, to facilitate the illustration, the illumination light incident on the center of the light illumination surface 502 will be described. That is, in the following description, each illumination light indicates the central beam (central axis) of each illumination light. Illumination light I1 emitted from an illumination optical system (not shown) impinges upon the prism 600. The illumination light I1 incident on the prism 600 undergoes total reflection at the selective reflection/transmission surface 620 of the prism 600. Illumination light I2 which has undergone total reflection at the selective reflection/transmission surface 620 is applied to the light illumination surface 502 of the DMD 500. The DMD 500 reflects the illumination light I2 applied to the light illumination surface 502 according to image information. Of the illumination light reflected by the DMD 500, illumination light I3 reflected in the z-direction is utilized as the light representing the image. The to illumination light I3 emitted from the DMD 500 impinges upon the prism 600, is transmitted through the selective reflection/transmission surface 620, and emitted to the projection optical system (not shown). The light impinging upon the projection optical system is projected to display the image.
The inclination of each micro-mirror varies in a see-saw-like manner about an axis parallel to the M-axis of FIG. 11(A), in response to an electric signal. In order that each pixel may be switched on and off by this inclination of the micro-mirror, there is a predetermined restriction to the incident angle of the illumination light incident on the DMD. That is, as shown in FIG. 11(A), when the optical path of each illumination light is projected onto the xy-plane parallel to the light illumination surface 502, the optical path of the illumination light I2 incident on the light illumination surface 502 is set so as to be directed obliquely downward to the right at an angle of approximately 45 degrees with respect to the central axis Lx parallel to the x-axis. Further, as shown in FIG. 11(D), in the plane including the illumination light I2 and the modulated light I3, the incident angle of the illumination light I2 incident on the light illumination surface 502 is set so as to be approximately 20 degrees.
In the prism 600, to satisfy the above restriction, as shown in FIG. 11(A), when the optical path of the illumination light I1 incident on the selective reflection/transmission surface 620 is projected onto the xy-plane, it is set so as to be parallel to the optical path of the illumination light I2. Thus, the illumination optical system emitting the illumination light I1 is usually arranged obliquely downward to the right at an angle approximately 45 degrees. As a result, in the conventional projector using a DMD as the electro-optic device, a space is needed not only in the horizontal direction but only in the vertical direction as the space for arranging the illumination optical system, making it difficult to reduce the thickness of the projector.
It is an object of this invention to provide a technique for reducing the size of a projector of the type which uses an optical modulator which modulates light by controlling the emitting direction of light applied to the light illumination surface according to the image information, as in the case of a DMD.
To achieve the above object, there is provided, in accordance with the present invention, a projector which may consist of:
an illumination optical system that emits illumination light;
an optical modulator having a light illumination surface that controls an emerging direction of the light applied to the light illumination surface in accordance with image information to modulate the light applied to the illumination surface;
a projection optical system that projects light emitted from the optical modulator; and
a prism provided between the optical modulator and the projection optical system.
The prism has a selective reflection/transmission surface that reflects the illumination light emitted from the illumination optical system to cause the illumination light to impinge upon the light illumination surface at a predetermined angle, and that transmits the light emitted from the optical modulator and that emits the light emitted from the optical modulator to the projection optical system. The selective reflection/transmission surface is formed such that, when the optical paths of the illumination light are projected onto a plane parallel to the light illumination surface, the inclination of the central axis of the illumination light emitted from the illumination optical system and impinging upon the selective reflection/transmission surface is different from the inclination of the central axis of the illumination light reflected by the selective reflection/transmission surface and impinging upon the light illumination surface.
In the projector of the present invention, the illumination optical system can be arranged such that, when the optical path of illumination light is projected onto a plane parallel to the light illumination surface, the inclination of the central axis of the illumination light emitted from the illumination optical system and impinging upon the selective reflection/transmission surface differs from the inclination of the central axis of the illumination light reflected by the selective reflection/transmission surface and impinging upon the light illumination surface, whereby the size of the projector can be reduced.
In the above projector, an arrangement may be adopted wherein the light illumination surface has a substantially rectangular contour. When an optical path of the illumination light is projected onto a plane parallel to the light illumination surface, the central axis of the illumination light reflected by the selective reflection/transmission surface and impinging upon the light illumination surface is inclined approximately 45 degrees with respect to a side of the contour of the light illumination surface.
In the conventional projector, when the optical path of the illumination light is projected onto a plane parallel to the light illumination surface, if the inclination of the central axis of the illumination light impinging upon the selective reflection/transmission surface is approximately 45 degrees with respect to the side of the contour of the light illumination surface, the largest space is required as the installation space for the illumination optical system. Thus, in the above projector, the reduction of the size of the apparatus is most effectively achieved.
In accordance with the present invention, there is provided a prism which reflects incident light from a first direction as reflected light in a second direction, and which transmits light incident from a third direction as transmitted light. The prism is provided with a light selective reflection/transmission surface that reflects the incident light from the first direction such that, when the optical paths of the incident light from the first direction and the reflected light are projected onto a predetermined plane, the inclination of the central axis of the incident light from the first direction is different from the inclination of the central axis of the reflected light.
By using the prism of the present invention in a projector, it is possible to obtain the same effect as that of the above projector.